In the industry and at power stations, cooling apparatuses, in which the medium to be cooled can be of a liquid or vaporous phase, are well known.
The known cooling apparatuses primarily used in large-size cooling systems of power stations comprise an air-cooled heat exchanger having a plurality of finned tubes connected in parallel, a plurality of air-side louvres, a mechanism for operating the louvres and means for actuating the operating mechanism according to temperature of the heat exchanger.
According to the current practice, the purpose of louvres used in cooling apparatuses is to control heat dissipation and to provide frost protection of heat exchangers. In the case of power station cooling apparatuses, controlling of heat dissipation is a secondary function, proven by the fact that louvres are generally not applied under climatic conditions where no frost risk prevails.
However, frost protection is extremely important because the finned tubes represent extremely large air side heat transfer surface, and heat dissipation is significant under air flow conditions. Thus, if the medium flowing in the finned tubes is overcooled or its flow stops, the medium could freeze within 1 to 2 minutes, leading to serious damage of the heat exchanger. By closing the louvres, the flow of cooling air can be reduced or totally stopped, thereby freezing can be avoided.
The number and size of louvres used in power station practice vary in an extremely wide range. As an example, in the case of a cooling tower associated with a 200 MW steam turbine, approximately 120 heat exchanger units, each having 60 louvres of 2.5.times.0.25 m size are required. The louvres are rotatably mounted adjacent to each other on frames in front of the heat exchangers in the direction of the cooling air flow.
In the current practice, these louvres are actuated by an electric motor or by hydraulic or pneumatic systems. The actuator is linked to the louvres by an operating mechanism, preferably a linkage of bars. In order to ensure a reliable protection of high value heat exchangers, a large number of temperature sensors, an associated transmitter and cable system as well as electric or electronic control equipment have to be used.
For ensuring a reliable protection complicated and costly detector and control system is required because it is difficult to identify unambiguously the temperature of medium flowing in heat exchangers. This depends on the location of the relevant finned tube in relation to the air flow direction, on the uniformity of the distribution of the medium to be cooled and also on other factors. Since the purpose of louvres is to ensure frost protection of heat exchangers, within a given group of finned tubes always the lowest temperature is to be determined for the medium, and the louvres are to be operated accordingly.
Another drawback of the known solutions is that it is difficult to ensure that the power required for operating the louvres is safely available, because the most critical condition from the aspect of frost risk could arise exactly at the time of operational disturbances and power breakdowns.